The business case of a cellular operator can be estimated by comparing the revenue received per user to the expense incurred per user. The infrastructure expense incurred per user can be approximated by base station cost (including the site and the backhauling) divided by the number of users served. Given that the base station cost will remain high (mainly due to site acquisition costs), it is understandable why there is an ongoing strive to serve as many users as possible by each base station.
Next generations of cellular communications will require several Mbits/sec per active user and at least hundreds of Mbits/sec per base station. Unfortunately, the need to provide very large capacity is not always matched by the availability of very high bandwidth. As a result, one would need to seek additional capacity multipliers, such as high spatial reuse factors to meet the required capacity. Assuming that channel bandwidths of 10-40 MHz and typical spectral efficiencies, spatial reuse factors of 10-30 will be required.
Typical base stations have spatial reuse factors of 3 due to sectorization. MIMO is a potential source of higher reuse, however in practice it is limited to an additional factor of 2 (due to the number of antennas in subscriber devices) and even this benefit is achieved only at high signal-to-interference ratios which are seldom achieved in cellular environment. Therefore, it would be advantageous to substantially increase the reuse factor by serving more users in parallel as proposed by the present invention.
Several technologies are available to increase the spatial reuse by generating multiple parallel data streams. The most obvious one is using multiple antennas with fixed narrow beams (or multibeam antennas implementing multiple beams with a common antenna aperture), each attached to its own radio modem. A more complicated solution is to form narrow beams by using phased array or digital beamforming techniques. At the extreme end, SDMA (Spatial Division Multiple Access) techniques create dynamically a set of beams according to the set of users being served at a given moment.
The techniques using fixed beams enjoy the simplicity of independent radio modem (including traffic scheduling) in each beam. In contrast, SDMA involves complicated logic for scheduling multiple transmissions to multiple users, as well as solving complicated multiple-user beamforming equations for each transmission burst.
The SDMA and subsectorization dimensions are similar in the sense that they enable obtaining higher throughput by serving multiple users at the same time. SDMA allows higher flexibility in selecting the sets of users to be served simultaneously, and lower mutual interference by directing nulls to other users when transmitting to each of the users that belong to such a selected set. The penalty paid is higher processing requirements for true SDMA (proportional to the cube of the number of simultaneous beams), as well as higher scheduling complexity.
The use of multiple transmit streams in parallel requires managing the interference among these streams. The interference management in cellular/sectorized deployment is achieved by planning the reuse scheme. Reuse planning is a well developed art for usual omnidirectional or 3-sector deployment. Reuse-3 and reuse-1 approaches are commonplace. Newer approaches such as using both reuse-3 (for edge-of-cell) and reuse-1 (for closer users) at the same cell, as well as Fractional Frequency Reuse (FFR), are being introduced. However, planning for deep subsectorization introduces new challenges due to the complex geometry of the interference environment.
Therefore, an adequate solution to the above described obstacles in implementing networks with high capacity base stations is required.
Several attempts were made to use sectorization with multiple narrow beams for access.
U.S. Pat. No. 6,748,218 assigned to REMEC Inc., discloses the use of multibeam antennas to achieve high spectral reuse within each base station. The publication is mainly concerned with a deployment that uses directional antennas at the subscriber side, and is less concerned with planning for the edge-of-cell, where the subscriber's directionality allows him to look at the desired base station while ignoring the base station at its back. This fact allowed the inventors to focus on the reuse among sectors within a base station, and for this two resources (frequency or polarization) in an A-B-A-B scheme would suffice. When concerned with interference among base stations, the focus is on sectors of two adjacent cells that are similarly oriented approximately along a bore axis of the two cells.
The MILTON project of the Canadian Research Centre (“CRC”) described the use of 24 beams with 5 GHz band transceivers for high capacity access. In their various publications, including U.S. Pat. No. 6,473,616, separate antenna was used for each beam. The reuse scheme demonstrated by CRC is concerned with a regular ABCDABCD scheme. The alignment of the antennas between cells was not chosen specifically, so illumination of cell edge regions by same channel does occur. Just as with U.S. Pat. No. 6,748,218, users are assumed to be directional and therefore illumination of a user by same channel from two different directions still works well.
Neither of the publications referred to above, solves the problem of interference management at edge of cell when the users use omnidirectional antennas as is common in mobile cellular systems, as done by the present invention.